The invention relates to a gas generator for a safety arrangement with a means for holding down propellant granule in a combustion chamber. A preferred field of application are gas generators for motor vehicle safety arrangements such as belt tensioners, airbags, roll bars or current supply cutoff units, which are filled with a corresponding propellant granule, preferably in tablet form, for the production of a defined gas volume in the case of activation.
Gas generators are to be used increasingly universally for various applications, e.g. in the driver""s or passenger""s airbag, in which differing gas volumes are made available with differing speed. The propellant granule is used for this in differing concentration, quantity and chemical composition. Fluctuations in the filling level, however, may cause relative movements of the propellant granule in the combustion chamber, a non-uniform distribution of the granule, and material abrasion on the propellant granule. Thereby, the required ignition- and pressure propagation is impaired. For these reasons, an arresting of the propellant granule is necessary. In addition, through an arresting, rattling noises of the granulate in the combustion chamber can be avoided.
As can be seen from the DE 42 27 547 A1, to balance out the filling level and to avoid relative movements, a filling body, designated in the publication as volume equalizing means, is used. This filling body, generally of elastic material, occupies the empty volume of greater or lesser size which is present in most cases and balances out fluctuations in level. The known filling body can, however, only balance out the filling level well within particular limits. In the case of greater level differences, the size or quantity of the filling body must be adapted accordingly. This increases the material costs, but in particular also the manufacturing expenditure, because means for recognizing the filling level and dosing the filling become necessary.
Furthermore, the use of filling bodies for arresting propellant granule in combustion chambers also proves to be unsuitable because in the case of combustion of the propellant, the chemical and physical characteristics and also the flow-dynamic characteristics at the outlet openings of the produced gas mixture are affected in a negative manner.
It is therefore an object of the invention to provide gas generators with a means for holding down propellant granule in a combustion chamber, in particular of gas generators for motor vehicle safety arrangements, which means limits the space for the propellant granule irrespective of the required filling level, and reliably arrests it.
The present invention provides a gas generator for a safety arrangement, in particular for a vehicle occupant restraint system, comprising:
a combustion chamber delimited by a combustion chamber wall and at least one combustion chamber cover;
propellant granule filled into the combustion chamber to a preselected filling level, and
a means arranged in the combustion chamber for holding down the propellant granule;
wherein the means for holding down the propellant granule comprises a disc-shaped holding face, the holding face being pressed into the combustion chamber and connected in a form- and force-fitting manner with the combustion chamber wall, and being pre-stressed for pressing elastically onto the propellant granule, thereby arresting the propellant granule at the preselected filling level and in a variable distance from the combustion chamber cover; and
wherein the holding face has grid holes having a diameter smaller than the propellant granule.
The holding face is to be understood here as a flat or corrugated structure, provided with holes, preferably a metallic fabric or a perforated metal sheet, the maximum grid hole diameter of which is smaller than the propellant granule, in order to ensure the arresting of the propellant granule in accordance with the invention. By means of the mesh width of such a grid or the number and size of the holes, in addition the outflow of the combustion gas into the empty space of the combustion chamber can be controlled and pre-filtered. The holding face is constructed such that a form- and force fitting connection to the combustion chamber wall is produced.
For the production of a form- and force-fitting connection between the holding face and the combustion chamber wall, various known solutions are available, such as for example a corresponding press-fit supported by a chamfering of the holding face edge or by spring rings. The arresting of the propellant granule takes place independently of the filling level by pressing in the disc-shaped holding face into the combustion chamber, the force and speed of pressing in being selected such that the propellant granule is pushed together accordingly and arrested, but a destruction of the propellant granule or abrasion is largely avoided. In addition, the holding face can be used for filtering the outflowing gas in order to avoid an outflow of combustion residues.
An advantageous further development of the invention consists in that through corresponding dimensioning of the form- and force-fitting connection and of the holding face in relation to the type of material and to the sum of the hole areas, the release of the arresting means is made possible on exceeding a predetermined threshold pressure. Thereby, undesirably high pressure peaks which arise through non-uniform deflagration or faulty propellant granule, can be intercepted.